The invention relates to a process for the separation of butanes and butenes by extractive distillation.
In the treatment of the C.sub.4 fraction resulting from steam cracking or catalytic cracking, the separation of butanes and butenes generally takes place downstream of the stages for the extractive distillation of 1,3-butandiene and the synthesis unit of methyl tert. butyl ether (MTBE). Thus, a C.sub.4 fraction is obtained, which is free from 1,3-butadiene and highly depleted in isobutene.
As butenes are valuable products, they are separated from butanes. Thus, the butenes can be separated into 1-butene and 2-butenes, or isomerized into isobutene, which is recycled in the MTBE synthesis unit. Butenes can also be dimerized by the DIMERSOL (registered trademark) process into products usable in gasoline. This dimerization is a process performed by homogeneous catalysis in which the catalyst is consumed. As the efficiency of the catalyst is proportional to its concentration in the charge, a paraffin-free charge leads to catalyst economies. Moreover, the butenes which have not reacted can be recycled if the butanes are previously removed from the charge.
It is known to separate butanes and butenes in a C.sub.4 fraction by extractive distillation. A prior art process (S. OGURA, T. ONDA, Advances in C.sub.4 Hydrocarbon Processing AlChE 1987 Summer National Meeting, Aug. 16-19, 1987) consists of carrying out extractive distillation with as the solvent dimethyl formamide (DMF) under pressure, solvent recovery taking place under atmospheric pressure and the purification of the butenes takes place under pressure. The extractive distillation column bottom product is fed into a column under atmospheric pressure, in which the butenes are desorbed and pass out at the top with part of the solvent, while the bottom product is constituted by virtually pure DMF. The top butenes are then passed to a column under pressure for purification. The solvent recovered at the bottom of the latter column and which still contains butenes is returned to the atmospheric column.
The disadvantage of operating according to this process is that most butenes in the charge must be distilled twice which, bearing in mind the necessary recompression of the butene vapours, leads to a high energy consumption.
In an earlier-dated application filed by the present applicant on 26.2.1991 under French national registration No. 91/02385, a description is given of a process for the separation of butenes and butanes, in which the charge containing the butanes and butenes to be separated is introduced into an extractive distillation column under pressure, where it is contacted with a polar solvent in which the butanes have a higher volatility than the butenes, the distillate passing out at the top consisting essentially of separated butanes. The residue collected at the bottom and mainly consisting of the solvent and the butenes, is passed into a column under pressure, where the bottom temperature is adjusted in such a way that the desorption of the butenes is not complete, which makes it possible to limit the column bottom temperature and consequently avoid the thermal decomposition of the solvent.
The solvent which passes out at the bottom and which contains a fraction of the butenes is passed into a purification column under a pressure close to atmospheric pressure and whose operation is regulated in such a way that the overhead vapor, mainly constituted by butenes, also contains a fraction of the solvent, which makes it possible to partly condense it in order to ensure the reflux of the column, while avoiding compression at this stage. The vapor distillate is passed, after compression, into the desorption column under pressure. The purified solvent passing out at the bottom is recycled to the extractive distillation column.